Cathodic lamp



2 '3 4 6 7 8 `9 /0 o/.scHA/eE cU/rEE/vr//IMPERESJ TEMPERATURE c. G. l-'oUND4 CATHODIC LAMP AFiled Feb. 2e, 1938,

N ov. 1, 1938.

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Inventor: Clncto G Found,

l-Is Attorney Patented Nov. 1, 1938 UNITED STATES 2,135,284 cA'rnoDro LAMP Clifton G. Found, Schenectady, N. Y., assignor to General Electric Company,` a corporation of New York Application February 26, 1938, Serial No. 192,808

3Claims.

'I'his application is a continuation-impart of my application Serial No. 103,566, led October l, 1936.

My invention relates to a gas-filled or discharge 5 lamp of the so-called cathodic type in which light generation is largely Aconned to a region closely adjacent the cathode.

It is an object of the invention to provide an electrode structure which will permit a greater luminous output for a given size of cathodic lamp than can be obtained with the structures heretofore employed. It is a further object to provide a high intensity cathodic lamp in which the light output is substantially insensitive to ambient temperature variations.

The novel features which I desire to protect herein will be pointed out with particularity in the appended claims. The invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following specification taken in connection with the drawing, in which Fig. 1 is a perspective view of a cathodic lamp embodying my invention; and Figs. 2 and 3 are graphical representations illustrating the improved characteristics which are realized by the use of the invention.

' The lamps with which my invention deals are of the thermionic discharge type employing an incandescible cathode and are conventionally constructed so that the distance between the cathode and the anode is comparable to the least dimension of the envelope. As a result of suchconstruction substantially all of the utilizable light energy is generated within a restricted radial distance of the cathode. It is, therefore, convenient to refer to such a light source as aV "cathodic lamp.

Cathodic lamps of the type with which my invention is particularly concerned comprise a sealed envelope defining a discharge space which contains suitable electrode structure, a vaporizable substance such as sodium and an inert or noncondensable gas such as neon. It will be understood, however, that although sodium and neon are particularly'referred to-in the following speciiication, it is possible, and my invention contemplates, that equivalent materials such as I nercury or caesium may be substituted for the former while other inert gases such as argon or krypton may be used instead ofv thelatter.

It is a function of the sodium or-equivalent vaporizable material to provide a source of substantial numbers of atoms which may be brought to a state of light radiation by impact with electrons of low energy content as is more fully explained in the following. The neon or other inert gas, on the other hand, has been found to serve a multiplicity of functions which add both to the convenience of operation and the efficiency 5 of the lamp. Thus, besides facilitating the initiation of a discharge between the lamp electrodes and the subsequent vaporization ofthe sodium, the neon provides atoms which serve as a deflecting means to lengthen the total path of the mov- 10 ing electrons, which would otherwise proceed almost directly through the eldof sparse sodium atoms to the anode. 'Ihis increase in the average path of travel of an individual electron greatly enhances the probability of vits encountering a l5 sodium atom in a light-emitting impact.

In addition to the two functions just described, neon is capable of a further service which makes it particularly useful in the cathodic type of lamp. It has been observed that the voltage drop in an 20 electron discharge tube having closely spaced electrodes is largely concentrated in a narrow sheath around the cathode which sheath may be less than 116 of a Acentimeter in thickness. It is within this sheath that emittedelectrons receive 25y v substantially all their energy, such energy being conveniently expressedl by giving the number of volts against which an electron of the given energy content is capable of moving. If the condition of operation of the lamp is such that an elec- 30 tronpassing from the sheath with an energy content above 16.5 volts collides with a, neon atom, lt may either ionize the same or excite it to luminosity, depending upon the exact energy values involved. The impinglng electron will not, of 35' course, be destroyed, but will continue its journey with its energy reduced by an amount which represents the ionization or excitation voltage of the neon. i

If such an electron of diminished energy, con- 40 veniently referred to as a secondary electron, retains, as is very likely to be the case, an energy of Vless than 5.1 volts and encounters a sodium atom, it will not be able to ionize the latter. Instead, a transfer of energy of, another type will 45 take place which is conventionally referred to as an excitation of the sodium atom. This isa. phenomenon which requires an electron energy of between 2.1 and 5.1 volts. An excited sodium atom is, generally speaking, capable of two further' 50 changes in condition; it may, if undisturbed for a brief period, return to its natural state, such return being accompanied with the production of light, or it may be converted into a sodium ion by a further impact with an electron having energy 56 micron.

of 3 or more volts. 'I'his latter conversion notonly fails to result in the production of light, but

Ialso eliminates' an excited sodium atom. This Y form ,of ionization by successive impacts with secondary electrons is conventionally referred as cumulative ionization. It tends to increase rapidly with increasing concentrations of excited atoms. In view of the above it is obviously desirable for the most efiicient production of light that all available secondary electron energy should be used in producing from unexcited sodium atoms a maximum .number of excited sodium atoms which, as before explained, are av potential source of light. Correlatively, the concentration or crowding of excited sodium atoms should be kept so low that cumulative ionization is substantvially avoided.A With the aboveprinciples in view it is believed that the nature of my invention now may be clearly understood. g

Referring to Fig. '1, I have illustrated an elongated sealed envelope I terminating at its base in a reentrant stem 2 provided with a press 3. A filling of a xed gas such as neon is provided in the envelope I and a quantity of sodium, or other vaporizable material, is deposited on the wall of the tube as shown at 4.

` Observations indicate that actual generation of light in a cathodic lamp takes place within a restricted radial distance of the cathode defined by the range within which substantially all secondary-electron energy will be absorbed. In the case of a sodium lamp, in normal operation, the entire tube may appear to be filled with a diffused sodium glow. This is because sodium resonance radiation, actually generated within the restricted region only, is continually absorbed and reemitted by. sodium atoms in the outlying space. Luminous emission from the tube is thus viewed externally as though it were taking place from the outer portion of 'the lamp.

The dimensions of the region of light generation are determined in part by the concentration of excitable atoms which in turn depends on the ratio of neon pressure to sodium pressure. The optimum condition which can be obtained is that at which the light generating region is coextensive with the volume of the bulb. It is not practical to assign an exact value for the best neon pressure to be employed, but this factor may be determined experimentally for any given tube construction. I have obtained most satisfactory resultsvwith a pressure of neon in the neighborhood of 400 microns of `mercury and a pressure of sodium of the order oi magnitude of one In order to prevent gradual condensation of the s odiuminto the relatively cold' base of the tube a mica disk 6 is provided which effectively divides the base froml the main body of the envelope. Through this disk lead-in wires are passed which at one end are sealed into the member 3 and at the other end serve to support the various electrodes. comprising a coiled ribbon ofsuitable metal, for example, nickel,.forming a helix which has its axis substantially coincident with the major axis of the envelope I. This anode is supported from the press 3 by a pair of conducting .rods 8 and 9, of whichfthe latter is provided with van external connection to permit a suitable potential to be impressed on the anode.

Concentrically arranged within the anode I provide a thermionic or incandescible cathode II comprising an elongated metal lament, either The anode 5 is shown as coiled or straight, coated with an electron emissive substance such as an alkaline earth metal oxide and supplied with heating current through,

surfaces of as great a longitudinal extent as the dimensions of the envelope permit. With a tubular envelope construction such as that shown, the

electrodes should extend lengthwise of the envelope for a distance substantially greater than the tube diameter and preferably commensurate with its length. For energizing the lamp, I have indicated an elementary circuit comprising a main transformer 22, a filament transformer 23 and a current. limiting resistor 24.

By analogy with accepted usage in other types of discharge lamps it has heretofore been deemed most advantageous, both for electron generation and for conservation of heating energy, to construct-the cathode member of a cathodic lamp in the form of a concentrated electron source, such as a closely coiled wire helix; I have found, however, that by using an elongated configurav tion such as that specified above, a greater luminous output may be obtained than has heretofore been deemed possible with lamps of the type in question. This unexpected result I attribute to the fact that a more advantageous distribution of emitted electrons is obtained. In other words, while the total number of electronsmay be unchanged, any unit volume of gas adjacent the cathode is subject to relatively less-`electron bombardment. Consequently, excitation of sodium atoms is disposed over so great a space that cumulative ionization as above defined is substantially prevented up to very high discharge currents. This means, of course, that a much greater amount of energy can-be utilized in light productionwithout cumulativeionization becoming the predominant energy absorbing factor. When the ratio of neon to sodium pressure also is so chosen that the region of secondary energy absorption extends radially as far as the wall of the envelope, optimum operating conditions exist.

- In previously constructed lamps of the type in question a maximum of light output has been obtained at a sodium' pressure such that less than all the secondary electron energy is absorbed. Any increase in pressure above the critical value magnies cumulative ionization to such an extent that a net loss rather than a net gain in light output is realized. As a consequence of this enforced suboptimal mode of operation changes in sodium pressure produced by variations in ambient temperature cause corous factors referred to in the foregoing. The' coordinates chosen are such that .the curves conditions.

show the variation in luminous output with changes in are current for different operating 'I'he lower curve, indicated as curve A, comprises the output characteristic of a cathodic lamp of known type in which the cathodev comprises a relatively concentrated coil. It will be seen that While the luminous output increases linearly with cu'rrent up to a value of about 11/2 amperes, above that value -the curve iiattens out to such an extent that little or no gain is realized by increasing the current input.

'I'he curve B shows the improvement obtained' in the operation of a cathodic lamp of identical construction with that of curve A by carefully selecting an optimum ratio of neon to sodium pressure which in the instance chosen operated at such temperature that the sodium;- vapor pressure was approximately one micron. However, in spite of thefact that a considerable in-` crease in luminous output is .realized by the change indicated a definite saturation eiect occurs at currents materially in excess of 5 amperes.

The curve C represents the results obtained by using( an electrode arrangement such as is shown in Fig. 1 in connection with an optimal neon pressure of about 400 microns. In the tests Irom whichv this curve was obtained the envelope and other structural parts were of precisely the same dimensions as were employed in connection with curves A and B. However, as a result of the increased gaseous volume traversed by the emitted electrons, a current intensity corresponding to'an ar'c current of 10 amperes was employed without producing any manifestation of saturation due to cumulative ionization. A luminous output Iof approximately 10,000 lumens was obtained in a lamp having a volumetric content of approximately 450 cubic centimeters and using a power input of. approximately.- 200 watts. To the best of my knowledge this is the first time that an output of this magnitude has been attained in a cathodic, lamp irrespective of the input employed.

A further advantage attendant upon the use of an extended rather than a concentrated cathode consists in the possibility of making a cathodio lamp which is substantially insensitive to variations in ambient temperature. l

Where a concentrated cathode is employed, a continuous variation of luminous output with temperature occurs, as is indicated, for example, by curve D in Fig. 3. In this curve a condition of constant current operation is assumed, the temperature being varied, for.example, by

changing the proportions or insulation of the discharge envelope. 'I'he rising portion of the,

curve to the left of the point X Vis due to the increase with temperature of the number ofsodium atoms .available for light excitation. 4The decreasing portion of the curve, on the other hand, I consider to be due to the destruction of excited sodium atomsby .cumulative ionization. With a concentrated cathode, the concentration of excitedsodium atoms in the vicinity of the cathode increases extremely rapidly with rising sodium pressure. Consequently cumulative ionization adjacent to the cathode becomes great enough to nullify thev favorable effect of the increased availability of sodium atoms even before enough atoms are provided to make it theoretically possible to utilize all available electhan' a net, gain in light output is realized above a certain point.

However, with an extended cathode such as.

characterizes my invention, excessive cumulative ionization will not set in until an amount of sodium vapor is present in excess of that required to utilize all available secondary electron energy in the excitation of light. When a completely adequate supplyl of vaporized sodium is present, a further increas'e in temperature and in sodium vapor pressure obviously cannot'increase the light generation. Consequently" Aa range of temperature exists in which constant light output will obtain at least up to the point where even the extended nature of the cathode cannot prevent excessive cumulative ionization from occurring. 'I'his range comprises, for example, the yregion from y to z on curve E of Fig.

`3. The point y indicates the temperature re quired to vaporize the least amount of sodium sufficient to produce maximum,A light excitation, i. e., 'to permit full utilization of available electron energy. 'I'he point z, on theother'hand, represents the temperature vat which excessive cumulative ionization begins to occur.

It is obvious that if the lamp can be operated so that its normal temperature of operation corresponds to the point l0 on curve E, the light output of the lamp may be substantially insensitive lto variations in ambient temperature (at least as long as such variations do not carry the operating temperature of the lamp above z or below il).

As a practical matter, this result can be obtained by selectingthe proportions and heat insulation of the lamp such that at the preferred operating currents, i. e. for discharge currents corresponding to the range of emcient operation .f may, for example, take the form of a transparent vacuum jacket such as is indicated at 25 in Fig. 1.

The form of lamp which I have described is exemplary only and it will be understood that various modications o f 'structure may be used. For example, the anode may be constituted of'a screen-like mesh rather than a spiral ribbon as illustrated and the cathode may lcomprise an velongated helical structure rather than a straight lament such as that shown in Fig, 1. I aim in the appended claims to cover all such variations of form and use as fall within the true spirit and scope. of the foregoing disclosure.

What I claim as new and desire to secureby Letters Patent of the United States is:

1. A cathodic lamp comprising an envelope which encloses a discharge-Supporting gas. a

vaporizable light-producing substance, and a pair of cooperating electrodes having mutually facing discharge-receiving surfaces of as great longitudinal extent as the major dimension of the envelope permits, the proportions and heat insulation of the envelope .'being such that at discharge currents within the range of eillcient operation of the lamp the amount of said substance maintained inthe vapor phase is appreciably above the least amount suflicient for maximum light excitation at 'such currents but below the value at which excessive cumulative ionization takes place, whereby the luminous output of the lamp is substantially insensitive to variations in ambient temperature.

2.` A cathodic lamp including an elongated tubular envelope which encloses a discharge-supporting gas, a vaporizable light-producing substance, and a vpair of cooperating electrodes hav` ing mutually facing discharge-receiving surfaces place, whereby the luminous output of said lamp is substantially insensitive to variations in ambient temperature.

3. A cathodic lamp comprising an elongated tubular envelope enclosing an inert gas, a quantity of sodium and concentrically arranged discharge electrodes having opposed discharge-receiving surfaces which extend longitudinally of the envelope a distance substantially greater than the diameter thereof, the proportions and heat insulation of the envelope being such as to main-v tain in the vapor phase during operation of the lamp a quantity of sodium intermediate between the least amount suflicient for maximum light excitation at the operating current of the lamp and.

the amount at which cumulative ionization becomes excessive, whereby the luminous output of the lamp is substantiallyv independent of variations in ambient temperature.

cm'roN G. FOUND. 

